In the manufacture of electronic devices from semiconductor materials, one technique of interest is the selective building up, followed by the selective removal, of various layers of different materials, so that the building up and removal processes result in specific electronic devices such as transistors, diodes, capacitors and the like.
One technique for removing layers of semiconductor or other materials from a given substrate is known as etching, which is the removal of a material following its interaction with another material generally referred to as the etchant. Etching techniques fall into two broad categories: wet etching which generally refers to techniques which take place in solutions or molten salts or other liquid materials; and dry etching which generally refers to the use of gases or plasmas to perform the removal which is desired.
Dry etching techniques are of particular interest in producing electronic devices because they generally exhibit better resolution and improved dimensional and shape control capabilities than do the various methods of wet etching. Accordingly, dry etching is favorably utilized where superior pattern control and delineation is required such as the processing of semiconductor wafers to form large scale integrated devices and integrated circuits.
Dry etching techniques can be used to micromachine mesas, isolation trenches, backside contact via holes, and other forms of pattern definition on thin films, substrates, or other materials.
One perennial candidate material for use in semiconductor devices--and which correspondingly requires etching in order to form certain of these devices--is silicon carbide (SiC). Silicon carbide has long been recognized as having certain favorable characteristics as a semiconductor material, including its wide bandgap, high thermal conductivity, high saturated electron drift velocity, and high electron mobility. To date, however, silicon carbide has not reached the commercial position in the manufacture of electronic devices that would be expected on the basis of its excellent semiconductor properties. This is a result of the difficulties encountered in working with silicon carbide: high process temperatures are often required, good starting materials can be difficult to obtain, certain doping techniques have heretofore been difficult to accomplish, and perhaps most importantly, silicon carbide crystallizes in over 150 polytypes, many of which are separated by very small thermodynamic differences. Accordingly, controlling the growth of single crystals or monocrystalline thin films of silicon carbide which are of a sufficient quality to make electronic devices practical and useful, has eluded researchers.
Recently, however, a number of developments have been accomplished which offer the ability to grow large single crystals of device quality silicon carbide, thin films of device quality silicon carbide, and to introduce dopants into silicon carbide, as required in the manufacture of many electronic devices. These include successful chemical vapor deposition (CVD) of both Beta-SiC and Alpha-SiC high quality thin films (epitaxial layers) on "off-axis" SiC substrates; improved sublimation growth techniques; and improved doping techniques, particularly ion implantation.
With the successes offered by these developments, an appropriate technique for etching silicon carbide is likewise desirable, for example in the production of mesa=type structures or any other structures in which etching is required.
A number of investigators have attempted to develop methods for etching silicon carbide under circumstances in which silicon carbide is used as a mask for a patterning process rather than as the active semiconductor portion of an electronic device. An early U.S. Pat. No. 3,398,033, to Haga discusses a method of etching silicon carbide using a mixture of oxygen and chlorine heated to between 1200.degree. and 1300.degree. C. Accordingly to Haga, this process partially deteriorates the silicon carbide, after which the remainder can be removed by a wet reaction in a mixture of hydrofluoric and nitric acids.
Yonezawa, U.S. Pat. No. 4,351,894, also discusses the use of silicon carbide as a mask material in manufacturing electronic devices from other semiconductor materials. According to Yonezawa, removal of silicon carbide is accomplished by either a plasma etching process using carbon tetrafluoride and oxygen, or by an electrolytic etching technique in which an electrolyte is selected from a mixture or perchloric acid, acetic acid and water; or from formic acid; or a mixture of sulfuric acid and water.
In a later patent, U.S. Pat. No. 4,560,642, Yonezawa discusses a slightly different technique for using silicon carbide as a mask material, but describes identical etching processes for removing the silicon carbide mask.
Yamazaki, U.S. Pat. No. 4,595,453, discusses a method of forming a semiconductor substrate, rather than a mask, which may be formed of silicon carbide. Yamazaki suggests using hydrogen fluoride gas (HF) as the reactive gas plasma for selectively or nonselectively etching the semiconductor silicon carbide substrate.
In the scientific literature, Lu et al., Thermal Oxidation of Sputtered Silicon Carbide Thin Films, J. Electrochem. Soc., 131, 1907 (1984), discuss masking techniques using amorphous silicon carbide films and using mixtures of tetrafluoramethane (CF.sub.4) and oxygen, as well as nitrogen trifluoride (NF.sub.3) as mask-removing reactive ion etching plasmas. The thin films described by Lu, however, are sputter deposited films of silicon carbide, a technique which results in either amorphous layers or partially polycrystalline layers which for practical purposes are amorphous. As is known to those familiar with semiconductor materials and their properties, such amorphous or polycrystalline materials are essentially useless for forming the active portion of semiconductor devices. Furthermore, Lu offers only a general discussion of the etching he reports carrying out.
A number of other researchers have studied potential techniques for etching silicon carbide and a selection of appropriate references has been cited herewith in the accompanying information disclosure statement.
By way of background, reactive ion etching is a procedure in which the material to be etched, sometimes called the target, is placed on a cathode in an electric field, and in the presence of a selected vaporized material. A potential is applied across the anode and cathode which is sufficient to ionize atoms and molecules in the vapor, as well as to produce some radicals. The potential difference accelerates positively charged ions in the vapor towards the target on the cathode. As these ions strike the material, they physically etch it away. In reactive ion etching, the vaporized material is selected to chemically react with the target material, thus enhancing the effects of the physical collisions.
In producing devices using such dry etching techniques, certain problems occur which must be addressed before successful results can be obtained. For example, when a gas like tetrafluoromethane (CF.sub.4) is used as the reactant gas, polymerization tends to occur among the fluorocarbon radicals formed, which in turn cause fluorocarbon compounds to deposit onto the surface being etched. These impurities, however, are undesirable whenever the etched surface is to be used for silicon carbide-based electronic devices. Because high quality ohmic contacts must eventually be made on the etched surface in order to produce workable devices, a smooth, chemically clean etch is imperative.
Accordingly, it is an object of the present invention to provide a method of dry etching monocrystalline silicon carbide which produces a smooth and chemically clean etched surface using plasma etching, reactive ion etching, or reactive ion beam etching processes; which method produces faster etch rates than have been possible to date for the dry etching of silicon carbide; and in which the etchant is efficiently broken into free radicals and for which all of the by-products of ionization are volatile; and using electrode materials which exhibit low sputter yield and which will react with the by-products of ionization so that they will not affect the etched surface.
The foregoing objects and advantages and other features of the invention will be set forth in the accompanying detailed description, in which preferred and exemplary embodiments are set forth, taken in conjunction with the accompanying drawings in which: